The process of this invention is generally applicable to the control of subsurface fluid movement through the reduction of porosity/permeability of geological formations. The process is especially suitable for use in the recovery of oil from hydrocarbon reservoirs.
In the recovery of oil from oil-bearing reservoirs, it is often possible to recover only a portion of the oil contained in the underground formation by the so-called primary recovery methods which utilize the natural forces present in the reservoir. Exceptionally poor oil recovery can be due to a number of factors including the breakthrough of an overlying gas body or an edge or bottom water to the production well by coning or channelling. This is particularly evident where reservoir heterogeneities such as fractures or high permeability streaks are selectively depleted of oil allowing the premature entry of adjacent gas or water into the production zone. In heavy oil fields, the relatively low viscosity and high mobility of a water phase allows such breakthrough even in homogeneous reservoirs by channelling or fingering of the water through the relatively immobile oil phase. Once highly water permeable channels are established, heavy oil production is lost.
A variety of enhanced recovery techniques, so-called secondary or tertiary recovery, have been employed in order to increase the recovery of oil from subterranean reservoirs. In one form of the enhanced recovery of oil, a drive fluid is injected under pressure into the oil reservoir through one or more injection wells to maintain, restore or produce formation pressure. The most widely used drive fluid is water, however, more complex aqueous systems, solvents and gases are also useful as drive fluids. Steam is often employed for heavy oils. The drive fluid is frequently introduced into the oil-bearing underground formation near the bottom of the formation at or above formation pressure, to displace oil in the reservoir. As the fluid moves through the reservoir, it drives or flushes the oil through the formation. An increased oil saturation develops ahead of the moving fluid and finally reaches the production well or wells.
Generally, an oil-bearing underground formation will consist of various regions having different permeabilities. Drive fluid moving through the reservoir preferentially moves to and through regions of higher permeability, fractures and the like. Drive fluids will pass predominantly through such channels bypassing regions of lower permeability and, thus, bypass oil contained in such lower permeability regions. This obviously reduces the sweep efficiency of the displacing medium.
The problems associated with enhanced oil recovery tend to be particularly acute when the oil is the highly viscous, so-called "heavy oil" which exists in the Lloydminster region in Canada and in certain reservoirs located in Alaska and Venezuela. In such heavy oil reservoirs, primary recovery and conventional waterflooding operations are sometimes estimated to produce as little as 4% to 8% of the oil contained in the reservoir.
Accordingly, new techniques to improve the extent and rate of heavy oil production would clearly represent a useful addition to the art.
Oil production may be enhanced through the application of a selective microbial plugging system, as disclosed in U.S. Pat. Nos. 4,460,043 and 4,561,500. However, microbial plugs can be susceptible to damage caused by high velocity fluid flows or large pressure drops across the plug, by thermal degradation in steam drive situations and the like or through the degradation of biological plugging materials by the indigenous microbial population. Thus, there is a need to provide improved plug stability to allow wider application of selective plugging.
As previously noted, the present process is generally applicable to the control of subsurface fluid movement. In this context, the process of this invention might be employed to reduce the permeability of an underground region adjacent to an aquifer, so as to reduce the probability of contamination of the aquifer.
Prior art techniques for reducing permeability in the vicinity of an aquifer often require the injection of noxious chemicals and/or polymers which may themselves contaminate the aquifer.
In summary, prior art techniques to reduce the permeability of subsurface geological formations can be susceptible to damage or large pressure drops across the plug, by thermal degradation in steam drive situations and the like or through the degradation of biological plugging materials by the indigenous microbial population and/or can require the use of noxious chemicals. It is an object of this invention to mitigate these disadvantages of the prior art.